Daily variations in physiologic and cognitive functions in human beings are driven by an endogenous circadian pacemaker. Human performance is further governed by the relative amounts of both chronic and acute sleep deprivation. Medical trainees, providing a significant amount of primary medical care in the U.S., are vulnerable to impaired performance and learning by virtue of schedules that demand long work hours. Surgical trainees, who are exposed to perhaps the greatest sleep deprivation and who are asked to perform critical tasks at adverse circadian phases, are a particularly important group. While much public policy debate has led to the mandate of reduced work hours, insufficient work has been done to determine the optimal work hours necessary to preserve top performance on critical tasks while preserving the educational opportunities for trainees. In June, 2003, ACGME work hour limitations loosely limit work hours to less than 80 hours a week and to no more than 24 hours of direct patient care. This proposal will attempt to work within that framework by testing surgical trainees on the following three schedules: 8-10 hour shift; 12-16 hour shift; and an on-call shift of 24-30 hours. Sleep will be characterized by actigraphy, diary self report, salivary cortisol assays and EEG recordings. Performance under these conditions will be measured using the MIST-VR laproscopic surgical simulator. Six critical tasks will be assessed for errors, efficiency and time on task and compared across the three schedules. [unreadable] [unreadable]